The present invention relates to inks for use in continuous ink jet printing systems and, more particularly, to a technique for matching the surface tension amongst all of the color inks used in a high speed, continuous ink jet printing system.
Continuous printing of variable color images at high speeds is applied to various kinds of substrates in the ink jet printing field. These substrates include uncoated, offset coated, and ink-jet coated papers. Traditionally, inks for continuous ink jet printers are obtained by dissolving water-soluble dyes in water or a mixture of water and water-soluble organic solvent(s).
In multi-color, high speed, continuous ink jet printing, the multiple colors are printed individually, typically using cyan, magenta, black and yellow. For example, to create a dark blue color on paper, the image will have cyan, magenta, and black, all in a specific area. Depending on the color, there will be various amounts of these inks. Thus, when the magenta is put down, it overlaps onto cyan. When black is put down, it overlaps onto both cyan and magenta.
This overlapping works well so long as there is not a great deal of intermingling or actual mixing of liquid drops on paper. When this happens, the color appears mottled, muddy or brownish. Also, the color changes with print speed. At slow speed, the inks have more chance to soak into the paper. At high speed, the inks tend to intermingle and the color shifts, usually appearing darker.
Another issue with high speed printing occurs at the boundaries between color patches. If, for example, a cyan area and a magenta area are immediately adjacent, one color may xe2x80x9cbleedxe2x80x9d into the other color.
In the existing art, these problems have been solved by drying each color ink before applying a subsequent color ink to the substrate. However, with multi-color printing, drying and paper handling for these multiple color applications can be costly.
It is seen then that there is a need for an improved technique for printing color inks which minimizes paper handling and costs, and maximizes image quality.
This need is met by the ink set according to the present invention, wherein the surface tension of every ink is configured to be approximately equal. Color-to-color bleed (when an unwanted blend of different colors occurs) and pullback (when different color inks do not properly blend) typically occur when surface tensions are unequal. With color-to-color bleed, an ink with a low surface tension will xe2x80x9cbleedxe2x80x9d or migrate into an area of high surface tension. Thus, if magenta ink has a lower surface tension than cyan, it will bleed into the cyan. The cyan area will then be a dark blue and the boundary between the colors will be blurred and feathery. On the other hand, if the cyan ink has a low surface tension, it will bleed into the magenta. This will happen regardless of which ink is printed first, since at high speed the ink is not completely dry as the paper passes under the subsequent color print head. This will cause the magenta area to appear blue, and again, the edges will be blurred and feathered.
The other phenomena that can occur, called xe2x80x9cpullbackxe2x80x9d, is also affected by unequal surface tensions of the inks. If an ink of very high surface tension is placed on top of an ink with very low surface tension, the high surface tension ink will pull back from the edges. This leaves an area of little or no ink where the high-surface tension ink should have been. For example, if low surface tension cyan is printed followed by a very high surface tension magenta, the magenta will pull back. This leaves an uneven, white area between colors which was not originally intended.
In accordance with one embodiment of the present invention, bleed and pullback are minimized by selecting an ink set having a particular print order, with each ink having its components selected to equalize the surface tensions of the inks. By equalizing the surface tensions, the percentage of density of the primary ink which can be printed increases, thereby increasing the secondary and tertiary color gamut.
Other objects and advantages of the invention will be apparent from the following description and the appended claims.